1. Field of the Invention
The present, invention relates to a heat exchanger utilized for a heat recovery steam generator in a combined cycle power generation plant or a convection section, and composed of a superheater, a reheater, an economizer and the like, of an outlet portion of a large-sized power generation radiant boiler In such a heat recovery steam generator or convection section, a plurality of tube banks are arranged in rows in a gas passage duct in a direction normal to a gas flow direction, and particularly, one in which the interval of a space between mutually adjoining tube banks is less than eight times the depth of a tube bank disposed on the upstream side and a resonance preventing baffle plate for preventing plural tube bank compound resonance is mounted in each of the tube banks. The depth is a distance from the central axis of the tube arranged on the most upstream side to the central axis of the tube arranged on the most downstream side as described hereinlater.
2. Prior Art
FIG. 6 is a schematic view showing a general structure of a multi-pressure type natural circulation heat recovery steam generator, in which exhaust gas from a gas turbine or the like first flows into a gas passage duct 1 of a natural circulation type heat recovery steam generator and then flows into a SCR (Selective Catalytic Reactor) 4 through a superheater 2 and a high-pressure evaporator 3. In the SCR 4, nitrogen oxide in the exhaust gas is removed. The exhaust gas discharged from the SCR 4 subsequently passes a high-pressure economizer 5, a low-pressure evaporator 6 and a low-pressure economizer 7 and is then subjected to heat exchanging operation with fluid inside the tubes constituting the respective tube banks. After heat exchanging operation, the exhaust gas is discharged into the atmosphere through a chimney, for example. A high-pressure steam and a low-pressure steam generated during the above process is utilized for a driving source of a steam turbine or an auxiliary heat source, for example. In FIG. 6, reference numeral 8 denotes a high-pressure steam drum and numeral 9 denotes a low-pressure steam drum
The respective tube banks of the multi-pressure type natural circulation heat recovery steam generator of the character described above are constituted by a number of tubes 10, as schematically shown in FIGS. 7 and 8, extending in a direction normal to the flow direction of the exhaust gas. The tube arrangement (array) or layout shown in FIG. 7 may be called an in-line array and the tube arrangement or layout shown in FIG. 8 may be called a staggered array. Usually, a tube pitch in the exhaust gas flow direction is represented by P.sub.L and a tube pitch in the direction normal to the gas flow direction is represented by P.sub.T.
The tubes 10 are disposed, as shown in FIG. 9, in an exhaust gas duct 1 which is comprised and separated from an external portion by duct side walls 11, a duct top wall 12 and a duct bottom wall 13.
When the tube banks are utilized for the natural circulation type of exhaust heat recovery steam generator, a finned tube 15 formed by securing a fin 14 to the tube 10, as shown in FIG. 10, may be utilized to enlarge the heat transfer surface area of the tube 10. It is a well known phenomenon that when an external fluid is flown in such tube banks, a vortex called the von Kerman's vortex is periodically generated with back flow in the tubes 10.
Generation frequency f.sub.K (H.sub.z) of such vortex is shown by an equation: EQU f.sub.x =S V/D (1)
(S: the Strouhal number (0.2 in case of a single tube, but different in case of tube banks in accordance with tube array); V: gap flow velocity (flow velocity at an interval between the tubes) (m/s); D: outer diameter (m) of the tube)
While there exists a natural vibration mode determined by the physical properties of the gas between the duct side walls normal to the gas flow direction and the tube axis, and its frequency f.sub.n (H.sub.Z) is represented as follows (in the case of gas, this frequency is called the frequency of standing wave oscillation). EQU f.sub.n =nc/2L (2)
(n=1, 2, 3--; c: speed of sound (m/s); L: width between duct side walls)
In the equation (2), the acoustic velocity c depends on a temperature of the gas of external fluid of the tube.
FIG. 11 shows the primary mode acoustic resonance (the primary mode) on the top side thereof and the secondary mode acoustic resonance (the secondary mode) on the bottom side thereof where a represents a node while b represents a loop.
As the load of the gas turbine changes, the temperature and the flow velocity of the exhaust gas flow from the gas turbine changes, and in a case where there is arranged a tube bank in which the generation frequency f.sub.K of the vortex caused by the back flow of the tube bank substantially accords with the frequency of standing wave oscillation f.sub.n, acoustic vibration, so-called acoustic resonance, is caused between the duct side walls in the direction normal to the fluid flow direction and the axial direction of the tube, which may result in generation of noise harmful to an environmental area, thus being not desirable. Furthermore, in a case where the resonant frequency generation is a value near the natural frequency of the structure, vibration in a direction horizontal to the duct side walls or the tube may be caused.
In order to obviate such defects, in the prior art, as shown in FIG. 12, baffle plates 16 for preventing the generation of the acoustic resonance are inserted in the tube bank 15 by dividing the duct width with a depth substantially equal to the depth of the tube bank. In FIG. 12, the staggered tube array is shown as one example and two baffle plates 16 are inserted to prevent the acoustic resonance phenomenon to the secondary mode from generating.
In this arrangement of the baffle plates 16, acoustic resonance can be prevented in the case of the single tube bank. However, as shown in FIG. 13, for example, in the case of a heat exchanger constituted by a plurality of tube banks, it has been experienced that such acoustic resonance cannot be prevented by merely inserting such baffle plates 16.
FIG. 14 is a graph showing the influence of the numbers of the rows of the tube banks 15 on the acoustic resonance, and in the graph, examples of 6 rows, 4 rows and 3 rows of the tube banks are shown. As can be seen from this graph, in the cases of 6 rows and 4 rows, there are portions at which sound pressures project, thus causing the acoustic resonance, but in the case of 3 rows, no resonance is caused. However, it has been found through experiment that the acoustic resonance is caused when such 3 row tube banks are arranged in plural numbers. Such acoustic resonance caused in the arrangement of a plurality of tube banks is called herein as multibank tubing compound resonance.
FIG. 15 is a graph representing the relationship between the interval of the gap portions of the plural number of tube banks and the sound pressure level raising components upon the generation of the acoustic resonance in a case where two tube banks are arranged, and the sound pressure level raising component is shown by the ordinate at the generation of the acoustic resonance and values obtained by dividing the interval of the gap between the tube banks by the depth of the tube bank arranged on the upstream side are shown by the abscissa. The depth of the tube bank is the distance from the central axis of the tube arranged on the most upstream side to the central axis of the tube arranged on the most downstream side.
As can be seen from FIG. 15, in a case where a value obtained by dividing the gap distance by the depth of the tube bank arranged on the most upstream side is less than 8 times, the raising of the sound pressure level is not observed, but in the case of less than 8 times, the raising of the sound pressure level is observed. In view of this phenomenon, it is considered that phenomenon substantially the same as that in the case of the single tube bank is caused in the case of the gap distance between the upstream side tube bank and the downstream side tube bank being less than 8 times the depth of the upstream side tube bank. In the case of the single tube bank, it has been shown through experiment that the acoustic resonance cannot be prevented in a case where a gap exists between the resonance-preventing baffle plates inserted into the tube bank.
In addition, it has been determined that the noise level will rise when the tube bank depth LA on the upstream side in FIG. 15 is equal and the gap LB of the cavity portion is short, and similarly, that the noise level will also rise when the tube bank depth LA on the upstream side is deep and when the gap LB of the cavity portion is equal.
Further, even in the case of the plural tube banks, these tube bank respectively behave as a single tube bank in the case of the gap or distance between the upstream and downstream side tube banks being more than 8 times of the depth of the upstream side tube bank.